Ignition coils are known for use in connection with an internal combustion engine such as an automobile engine. Ignition coils typically include a core around which is wound a primary winding. A secondary winding is wound around a secondary winding spool to surround the primary winding such that a high voltage is induced on the secondary winding when an electric current applied to the primary winding is stopped. One end of the secondary winding is a low-voltage end which is connected to a ground terminal while the other end of the secondary winding is a high-voltage end which is connected to a high-voltage terminal which is in electrical communication with a spark plug, thereby delivering a spark-generating current to the spark plug. Delivering the spark-generating current to the spark plug results in the first several hundred turns of the high-voltage end of the secondary winding being subjected to a high current pulse. Unless the high current pulse is properly dealt with, the secondary winding may be undesirably affected by the high current pulse.
The high current pulse is most easily limited by using a high resistance spark plug. High resistance spark plugs were common in less-recent internal combustion engines, and consequently, the high current pulse was not of sufficient magnitude to need to be factored into the secondary winding design. However, more modern internal combustion engines are commonly utilizing lower resistance spark plugs in order to boost energy from the coil, thereby resulting in higher voltages and making the high current pulse a magnitude that may not be compatible with the secondary winding.
Another way to deal with the high current pulse is to wind the secondary winding in a segmented winding strategy where a plurality of axially spaced ribs on the secondary winding spool forms a plurality of channels therebetween. Consequently, the secondary winding is divided into segments where the potential difference is minimized between the outermost windings of a given segment and the innermost windings of the given segment. As a result, the potential difference is kept to an acceptable level between the outermost winding and the innermost winding at the high-voltage end of the secondary winding that is subjected to the high current pulse. An example of a secondary winding using such a segmented winding strategy is shown in United States Patent Application Publication No. US 2013/0291844 to Skinner et al., the disclosure of which is incorporated herein by reference in its entirety.
While the segmented winding strategy of United States Patent Application Publication No. US 2013/0291844 to Skinner et al. may be effective for dealing with the high current pulse, it may be desirable to use a progressive winding strategy where the secondary winding is wound uninterrupted around the secondary winding spool. Using the progressive winding strategy may be more desirable than the segmented winding strategy because the axial spaced ribs used to implement the segmented winding strategy add stress to the secondary winding spool and thereby require the secondary winding spool to be made of special material under some circumstances. The progressive winding strategy may be less costly to manufacture due at least in part to requiring less costly material, furthermore, the progressive winding strategy may allow the ignition coil to be made more compact which is particularly important when the ignition coil is a plug-top coil. However, since the progressive winding strategy is wound uninterrupted around the secondary winding spool, the potential difference between the outermost winding and the innermost winding is greater than in the segmented winding strategy. In order to minimize the potential difference between the outermost winding and the innermost winding in the progressive winding, the secondary winding may be increased in axial length, thereby decreasing the thickness of the secondary winding by spreading the number of windings over a greater length, however, this may not be possible to do while maintaining a desired packaging size of the ignition coil. An example of a secondary winding using such a progressive winding strategy is shown in U.S. Pat. No. 6,556,118 to Skinner et al., the disclosure of which is incorporated herein by reference in its entirety.
What is needed is an ignition coil which minimizes or eliminates one or more of the shortcomings as set forth above.